Resumen

ECHEVESTE, Horacio  y  FERNANDEZ, Raúl. Fragmentation during outflow in the Las Lajas Jgnimbrite, Chon Aike Formation, Deseado Massif. Rev. Asoc. Geol. Argent. [online]. 2012, vol.69, n.1. ISSN 0004-4822.

The petrographic analysis of the vitrophyre that makes up the basal layer of the Las Lajas rhyolite ignimbrite lead to the identification of textures that indicate a flow of the set of phenocrysts-matrix as a response to the load pressure and to a ground slope (~20°). The majority of the phenocrysts, feldspar, quartz, and biotite are subhedral and anhedral crystals which have fragmented prematurely during the magma vesiculation during eruption. After the "sedimentation" of the phenocrysts and the molten material that formed the matrix, the majority of the phenocrysts fractured once again, possibly due to the thermal contraction and expansion and, in some cases, self-collision. The pending laminar flow below the set of molten crystals in the base of the unit generates a shear strain that displaces the fragments of the brittle broken crystals. At the same time, the biotite breaks along the cleavage planes and has a ductile deformation due to shocks with other solid crystals and by adapting to the matrix's deformation flow direction. The deformation due to the vitrophyre was produced at a temperature above 730 °C (glass transition temperature for calc-alkaline rhyolitic melt) and with a viscosity for the phenocrysts-matrix below the 8.64 E+12 PS s. This fragmentation mechanism due to flow would be the equivalent to the one produced in rhyolitic lava and lava domes and is similar to that produced in the submagmatic flow, although with a higher strain rate that involves cataclasis of crystals.

Palabras clave : High grade ignimbrite; Vitrophyre; Broke crystals; Rheomorphism; Jurassic.

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